In such a process, nests of patterns made of wax or the like are provided with a stable ceramic layer of several mm in thickness by applying a plurality of dip coatings. The individual layers are dried or cured individually. Coarse refractory powder or sand is sprinkled onto the individual wet layers as a binding link to the next dip coating. The shells are fired after dewaxing, after which they can be used for casting while warm or after cooling.
Lost-wax mold shells which are prepared according to the above-described process and are used for open casting in air react with the ceramic layer facing the metal due to the formation of a skin decarburization in castings consisting of unalloyed and alloyed steels as well as by skin decarburization and pitting in the case of castings of 13% to 17% chrome steels and of steel grade 17-4 PH. Pitting may also occur in stainless and heat-resisting steels.
Attempts have been made to avoid the above-mentioned disadvantages by casting the lost-wax mold shells under vacuum, in containers in the absence of air, or under reducing protective gases or reducing protective materials.
The cooling of the mold shell under a protective gas is extremely expensive, and it becomes increasingly expensive as the mold assumes larger dimensions, and it does not always lead technically to tile goal of avoiding decarburization and pitting.
The addition of reducing materials, e.g., graphite, pyrolytic graphite and/or meltable metal compounds to avoid decarburization and pitting in the various steel alloys has been known.
British Patent No. 672,535 recommends the addition of coke, activated carbon, activated Al.sub.2 O.sub.3 or SiO.sub.2 or a metal, e.g., nickel or aluminum, to prevent skin decarburization during the casting of parts cast according to tile lost-wax process in a compact mold.
Difficulties due to the combustion of the carbon and its compounds and destruction of tile shell mold have systematically occurred especially when carbon, carbon-containing substances and/or metal compounds were added to the shell ceramic.